js-on-sx: String fixes — fromCodePoint, multi-arg indexOf/split/lastIndexOf, matchAll, constructor, js-to-string dict

- String.fromCodePoint added (BMP + surrogate pairs)
- indexOf/lastIndexOf/split now accept optional second argument (fromIndex / limit)
- matchAll stub added to js-string-method and String.prototype
- String property else-branch now falls back to String.prototype (fixes 'a'.constructor === String)
- js-to-string for dict returns [object Object] instead of recursing into circular String.prototype.constructor structure
- js-list-take helper added for split limit

Scoreboard: String 42→43, timeouts 32→13, total 162→202/300 (54%→67.3%). 529/530 unit, 148/148 slice.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/js/runtime.sx

@@ -1169,7 +1169,7 @@
       ((= v false) "false")
       ((= (type-of v) "string") v)
       ((= (type-of v) "number") (js-number-to-string v))
-      (else (str v)))))
+      (else (if (= (type-of v) "dict") "[object Object]" (str v))))))
 
 (define
   js-template-concat
@@ -1903,7 +1903,15 @@
               (char-code (char-at s idx))
               0))))
       ((= name "indexOf")
-        (fn (needle) (js-string-index-of s (js-to-string needle) 0)))
+        (fn
+          (&rest args)
+          (if
+            (empty? args)
+            -1
+            (js-string-index-of
+              s
+              (js-to-string (nth args 0))
+              (if (< (len args) 2) 0 (max 0 (js-num-to-int (nth args 1))))))))
       ((= name "slice")
         (fn
           (&rest args)
@@ -1927,7 +1935,16 @@
               (js-string-slice s lo (min hi (len s)))))))
       ((= name "toUpperCase") (fn () (js-upper-case s)))
       ((= name "toLowerCase") (fn () (js-lower-case s)))
-      ((= name "split") (fn (sep) (js-string-split s (js-to-string sep))))
+      ((= name "split")
+        (fn
+          (&rest args)
+          (let
+            ((sep (if (= (len args) 0) :js-undefined (nth args 0)))
+              (limit
+                (if (< (len args) 2) -1 (js-num-to-int (nth args 1)))))
+            (let
+              ((result (js-string-split s (js-to-string sep))))
+              (if (< limit 0) result (js-list-take result limit))))))
       ((= name "concat")
         (fn (&rest args) (js-string-concat-loop s args 0)))
       ((= name "includes")
@@ -2042,6 +2059,17 @@
                     (= idx -1)
                     nil
                     (let ((res (list))) (append! res needle) res))))))))
+      ((= name "matchAll")
+        (fn
+          (&rest args)
+          (if
+            (empty? args)
+            (list)
+            (let
+              ((needle (js-to-string (nth args 0))))
+              (let
+                ((loop (fn (start acc) (let ((idx (js-string-index-of s needle start))) (if (= idx -1) acc (let ((m (list))) (begin (append! m needle) (dict-set! m "index" idx) (loop (+ idx (max 1 (len needle))) (begin (append! acc m) acc)))))))))
+                (loop 0 (list)))))))
       ((= name "at")
         (fn
           (i)
@@ -2068,7 +2096,14 @@
             -1
             (let
               ((needle (js-to-string (nth args 0))))
-              (js-string-last-index-of s needle (- (len s) (len needle)))))))
+              (let
+                ((default-start (- (len s) (len needle)))
+                  (from
+                    (if (< (len args) 2) -1 (js-num-to-int (nth args 1)))))
+                (js-string-last-index-of
+                  s
+                  needle
+                  (if (< from 0) default-start (min from default-start))))))))
       ((= name "localeCompare")
         (fn
           (&rest args)
@@ -2166,6 +2201,15 @@
       ((not (= (char-at s (+ si ni)) (char-at needle ni))) false)
       (else (js-string-matches? s needle si (+ ni 1))))))
 
+(define
+  js-list-take
+  (fn
+    (lst n)
+    (if
+      (or (<= n 0) (empty? lst))
+      (list)
+      (cons (first lst) (js-list-take (rest lst) (- n 1))))))
+
 (define
   js-string-split
   (fn
@@ -2306,7 +2350,13 @@
               (js-string-method obj "toLocaleUpperCase"))
             ((= key "isWellFormed") (js-string-method obj "isWellFormed"))
             ((= key "toWellFormed") (js-string-method obj "toWellFormed"))
-            (else js-undefined)))
+            (else
+              (let
+                ((proto (get String "prototype")))
+                (if
+                  (and (dict? proto) (contains? (keys proto) key))
+                  (get proto key)
+                  js-undefined)))))
         ((= (type-of obj) "dict")
           (js-dict-get-walk obj (js-to-string key)))
         ((and (= obj Promise) (dict-has? __js_promise_statics__ (js-to-string key)))
@@ -3028,6 +3078,35 @@
   js-string-from-char-code
   (fn (&rest args) (js-string-from-char-code-loop args 0 "")))
 
+(define
+  js-string-from-code-point-loop
+  (fn
+    (args i acc)
+    (if
+      (>= i (len args))
+      acc
+      (let
+        ((cp (floor (js-to-number (nth args i)))))
+        (if
+          (< cp 65536)
+          (js-string-from-code-point-loop
+            args
+            (+ i 1)
+            (str acc (js-code-to-char (js-num-to-int cp))))
+          (let
+            ((hi (+ 55296 (floor (/ (- cp 65536) 1024))))
+              (lo (+ 56320 (modulo (- cp 65536) 1024))))
+            (js-string-from-code-point-loop
+              args
+              (+ i 1)
+              (str
+                (str acc (js-code-to-char (js-num-to-int hi)))
+                (js-code-to-char (js-num-to-int lo))))))))))
+
+(define
+  js-string-from-code-point
+  (fn (&rest args) (js-string-from-code-point-loop args 0 "")))
+
 (define
   js-string-from-char-code-loop
   (fn
@@ -3058,8 +3137,15 @@
 
 (dict-set! String "name" "String")
 
+(dict-set! String "fromCodePoint" js-string-from-code-point)
+
 (define Boolean {:__callable__ (fn (&rest args) (if (= (len args) 0) false (js-to-boolean (nth args 0))))})
 
+(dict-set!
+  (get String "prototype")
+  "matchAll"
+  (js-string-proto-fn "matchAll"))
+
 (dict-set! Boolean "length" 1)
 
 (dict-set! Boolean "name" "Boolean")